I checked into my flight online in Minneapolis and instead of printing my boarding pass I had it sent to my iPhone. From the received QR code (kind of like a square barcode) I could simply place my screen against a scanner to get through security, into the Crown Room, and onto the plane.
But necessary? Only minimally green and, thus far, not really a practical improvement over the paper boarding pass.
Beware of cool solutions that are just that cool but overkill.
Sometimes simpler really is better.